RFR(XXS) 8186797: cardtable_rs in g1CollectedHeap::initialize() defined, but never used

[Zhengyu Gu]

Please review this one line cleanup to remove unused cardtable_rs.

It seems to be a leftover of refactoring 
G1RegionToSpaceMapper::create_mapper() to create_aux_memory_mapper().


Bug: https://bugs.openjdk.java.net/browse/JDK-8186797
Webrev: http://cr.openjdk.java.net/~zgu/8186797/webrev.00/

Test:

hotspot_gc on Linux x64 with fastdebug and release build


Thanks,

-Zhengyu